The present invention concerns temperature control systems which control the air temperature inside vehicle passenger compartments, most typically automotive vehicle passenger compartments. Typically, such temperature control systems operate on the basis of negative-feedback control action and include a command transducer which is adjusted by a passenger or driver to select a desired temperature, a passenger-compartment temperature sensor which senses the temperature inside the passenger compartment, and also a heat-exchanger temperature sensor which senses the temperature at or near the surface of a heat exchanger through which heat-exchange fluid flows for heating or cooling the air entering the passenger compartment. In such systems, the signal from the command transducer is compared with the feedback signal developed from the two differently located temperature sensors, and in dependence upon the discrepancy between the two signals the heat-exchanging action of the heat exchanger is automatically adjusted.
In the prior art, the feedback signal developed from the passenger-compartment temperature sensor and the heat-exchanger temperature sensor is made mainly dependent upon the signal from the passenger-compartment temperature sensor and less dependent upon the signal from the heat-exchanger exchanger temperature sensor. For example, the signals from the two temperature sensors may be algebraically summed with different respective weighing factors, to produce the actual feedback signal of the temperature control system. Conventionally, the dependence of the system feedback signal on the passenger-compartment temperature signal is five to eight times as great as its dependence on the heat-exchanger temperature signal. This difference in weighting factors is important for the stability of the negative-feedback temperature control system. If the weighting factor applied to the signal from the heat-exchanger temperature sensor is made too low, or if no heat-exchanger temprature sensor is used at all, then system operation dependent upon the passenger-commpartment temperature sensor alone exhibits periodic departures from the commanded temperature, these departures being of considerable magnitude.
On the other hand, the effect upon the system feedback signal of the heat-exchanger temperature signal leads to steady-state error in system performance, such steady state error becoming of unacceptably great magnitude, for example, in the case of heating systems used in winter where a high temperature is necessary for the heat exchanger. Because in extreme cold the heat-exchanger temperature must be made quite high to implement a comfortable passenger-compartment temperature, the contribution of the heat-exchanger temperature signal to the system's feedback signal tends to produce a disproportionately great increase in the system's feedback signal. I.e., the high temperature of the heat exchanger is, so to speak, confused with the passenger-compartment temperature, with the result being a passenger-compartment temperature considerably lower than actually commanded by the driver or passenger.
This can be explained with regard to a numerical example. In summer the typical temperature of the heat exchanger will be ca. 20.degree. C. In contrast, in winter the heat-exchanger surface temperature will be ca. 60.degree. C. Assume that the system's temperature feedback signal corresponds to the sum of the sensed passenger-compartment temperature and one-fifth the sensed heat-exchanger temperature. Accordingly, a 1.degree. C. rise in the temperature sensed by the passenger-compartment sensor increases the system's feedback signal by five times the amount which would result from a 1.degree. C. rise in the temperature of the system's heat exchanger. If now the temperature of the heat exchanger changes from 20.degree. C. to 60.degree. C., i.e., a rise of 40.degree. C., the effect upon the system's feedback signal is the same as that of 8.degree. C. rise in the interior temperature of the passenger compartment. Thus, as between the two heat-exchanger temperatures, and assuming the selected temperature to be the same in both cases, the system's feedback signal tends to simulate a passenger-compartment temperature increase of 8.degree. C. If during summer operation, i.e., with the heat-exchanger temperature at 20.degree. C., the system is so designed that the selected temperature is accurately implemented by the system, then during winter operation, i.e., with the heat-exchanger temperature of 60.degree. C., and assuming the same interior temperature to have been selected, the system will develop a steady-state error of about 8.degree. C. Furthermore, The amount of the steady-state error increases with increases of selected temperature.
This low accuracy of such passenger-compartment temperature control system is less than satisfactory, and quite unsatisfactory in the case of vehicles destined for localities having extremes of temperature, e.g., very hot days and very cold nights. Moreover, and as explained above, it is not appropriate to eliminate the accuracy-reducing effect of the heat-exchanger temperature sensor by eliminating the sensor itself, because the result is then unstable system behavior.